with Samba 3.6.8 "net" behaves wrong:
"net ad join" now asks for the password of user "(null)" instead of "root".
"net ad join -U domainadmin" crashes after entering the password
I think it's caused by bug 9330 (https://bugzilla.samba.org/show_bug.cgi?id=9330).
The ("null") is caused by a missing LOGNAME=root variable after a "su -l root"
Should be fixed.
Closing out bug report.
Please feel free to re-open if it's still an issue.